<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>01</title>
<style type="text/css">
.d1 {
	border: 3px solid green;
	width: 400px;
	height: 400px;
	margin: auto;
}

.d2 {
	border: 3px solid black;
	width: 200px;
	height: 200px;
}

.d3 {
	border:3px solid red;
	width: 50px;
	height: 50px;
}
</style>
</head>
<body>
	<div class="d1">
		<div class="d2">
			<div class="d3"></div>
		</div>
	</div>


</body>
</html>